CHEMSUSCHEM
COMMUNICATIONS
which resulted in the cessation of the catalytic activity (Fig-
ure S4). Catalyst removal was performed by filtration at the re-
action temperature (413 K). In additional experiments, AuI and
AuIII species, introduced into the solution as HAuCl4 or
(CH3)(PPh)3Au, resulted inactive for the O2-assisted coupling of
benzene (Table 1). We infer, thus, that nanoparticulate Au in
the Au/TiO2 sample is responsible for the observed catalytic
performance in the benzene coupling. In agreement with this
postulation, Au nanoparticles on other supports such as Al2O3
or ZnO also showed high catalytic activity for the formation of
biphenyl, although the resulting TOF and TON values were
slightly lower than those provided by the Au/TiO2 catalyst
(compare entries 1, 9, and 10 in Table 1). Some reports have
shown the unusual oxidation performance of Au if the size of
the nanoparticles is <2 nm.[34] However, although the number
of nanoparticles smaller than 2 nm is greater in Au/Al2O3 than
in Au/TiO2 (the fraction of these sites in our sample is rather
low), the activity per exposed Au center is lower for the
former, which suggests that highly dispersed Au sites are not
particularly active for the O2-assisted coupling of nonactivated
arenes.
such as horseradish peroxidase are active for the coupling of
aromatic alcohols through a rather benign approach (H2O2 as
the oxidant) but selectivities to the dimeric alcohols are in gen-
eral low.[35]
The observation that a mixture of all possible regioisomers
is obtained with some reactants suggests the likelihood of
a radical reaction mechanism, which is a common pattern in
the CÀH bond scission of arenes (e.g., by using stoichiometric
amounts of FeCl3).[36] As we do not observe any coupling activi-
ty if the experiments are performed under a pressure of N2
rather than O2, we postulate a homolytic sequestration of the
H moiety by an AuÀO adduct, as in other oxidative Au-cata-
lyzed transformations.[37,38] The performance of Au is consis-
tently striking in this type of reaction. The synthesis of asym-
metric biaryls through Au-catalyzed aerobic cross-coupling re-
actions would certainly be of interest.[39]
In summary, our results demonstrate that catalytic amounts
of Au in the form of small nanoparticles on TiO2, Al2O3, or ZnO
are able to activate ArÀH bonds directly in O2 and yield
a number of biaryls selectively at moderate temperatures with-
out the need to use additives such as iodine, acids, or bases to
facilitate multiple turnovers and in the total absence of a sol-
vent. This synthetic protocol is en route towards a zero-waste
production of biaryls.
In a subsequent series of experiments, the Au/TiO2 catalyst
was evaluated for the coupling of various substituted arenes
(Table 2). The biaryl formation took place selectively with aro-
Experimental Section
Table 2. Catalytic performance of Au/TiO2 for the O2-assisted coupling of
various arenes at 12 bar.
Catalyst preparation
[f]
Substrate
T[a]
[K]
Au/arene[b]
TOF[c,d] TON[e] SBiaryl
[%]
The catalysts 1 wt% Au/TiO2, 1 wt% Au/Al2O3, and 1 wt% Au/ZnO
were used as received from STREM (AUROliteTM catalysts). The cat-
alysts can also be prepared by following a deposition–precipitation
method from HAuCl4 as described elsewhere.[31] Pt/Al2O3, Rh/TiO2,
Ni/TiO2, and Pd/TiO2 samples were prepared by the incipient wet-
ness technique. H2PtCl6 (hexahydrate, Aldrich, >37.5% Pt), Ni(NO3)2
(hexahydrate, Fluka, >98.5%), RhCl3 (Aldrich, Rh content 40%),
and PdCl2 (Aldrich, 99%) were used to impregnate TiO2 (Degussa
P-25) in water. As an example, an aqueous solution (2 mL) that
contained H2PtCl6·6H2O (13.27 mg) was contacted with TiO2 (1 g)
to prepare the 0.5 wt% Pt/TiO2 catalyst. After perfect mixing of the
corresponding slurries, samples were dried at 373 K for 12 h. Some
samples were reduced in a flow of H2 at 723 K for 3 h before reac-
tion as specified in Table 1. The bimetallic Au-Pd/TiO2 catalyst was
synthesized by impregnation of the Au/TiO2 catalyst with a solution
of PdCl2 to have a final Au/Pd molar ratio of approximately 1. This
catalyst was washed thoroughly with deionized water and dried at
373 K for 12 h before reaction.
[mol ratioꢀ100] [hÀ1
]
benzene
toluene
p-xylene
413 0.022
373 0.026
373 0.030
382
176
18
230
206
71
8
88
99
98
98
98
99
99
80
1,2,4-trimethylbenzene 373 0.034
6
chlorobenzene
nitrobenzene
phenol
413 0.032
413 0.035
373 0.027
368
176
85
175
336
[a] Reaction temperature. [b] Amount of catalyst as (mol of Au)/(mol of
arene)ꢀ100. [c] Turnover frequency (mol of benzene converted)/[(mol of
metal)ꢀh]. [d] Typical relative standard deviations are 10–12%. [e] Turn-
over number (mol of benzene converted)/(mol of metal). [f] Selectivity to
the substituted biaryl; some reactions produce a mixture of regioisomers
(see the Supporting Information, Table S3). Turnovers calculated per
metal atom on the external surface of the nanoparticles.[33] Experimental
details are provided in the Supporting Information.
matic compounds that incorporated alkyl, ÀCl, ÀNO2, and ÀOH
groups, though at variable rates, which depended on steric
factors, nature of the substituent, and reaction temperature.
Recently, the aerobic coupling of electron-deficient arenes
such as nitrobenzene was performed successfully with soluble
Pd salts in the presence of trifluoroacetic acid,[12] but the
number of turnovers achieved by this route is rather low (ap-
proximately 5). The Au/TiO2 catalyst proposed here, in contrast,
works under neutral conditions and allows the completion of
at least 175 turnovers in the coupling of nitrobenzene, which
opens the door for an environmentally friendly and highly
atom-efficient route for the synthesis of biaryls. Other catalysts
Oxidative coupling of arenes
Catalytic testing was performed in a reinforced glass reactor (2 mL
volume) equipped with a temperature and pressure control and
stirred magnetically. Before each experiment, all the material was
washed with abundant acetone and dried at 383 K for >5 h. The
presence of acetone or other polar molecules such as ethanol in
the reaction mixture must be avoided completely for good replica-
tion of the results. In a typical experiment, the aromatic compound
(891 mg) was placed in the reactor together with catalyst (30–
70 mg) and dodecane (9 mg) as an internal standard. Reactants
were obtained from Sigma–Aldrich with purities above 99% and
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 4
&
3
&
ÞÞ
These are not the final page numbers!